I'm modifying a solid state guitar pre-amp to be a hybrid. Op amps don't sound very good when you overdrive them so I have replaced some op amps with ECC83 tube sections. The amp has a hybrid power amp, so modifying it for the plate voltage for the ECC83's was not to hard.

The pre-amp sounds much better when overdriven now but I'm having trouble with high pitch squealing. I figure it is oscillation. If I keep the post gain turn down to the point of almost off, I get no squeal and I can adjust the pre gain to give me clean or distorted tones at low volumes. When I turn up the post gain, high pitched squealing starts and changes pitch as I move the post gain pot changing from a squeal to a screech as the volume increases. This happens with the pre gain at any setting, even turned all the way down. If I turn the treble pot all the way down, the squealing stops and I can adjust the pre and post gains to get clean or distorted at any volume, but it sounds like I have the treble turned all the way down.

Originally I did not use the grid resistors TR2 and TR6. I added them to try to stop the oscillations, but they did not help.

Anyone have any ideas?
Thanks.
Scott

Update. I was checking the amp to see if there was anything else I could add before I posted this and I found an interesting fact. With the pre gain turned up to max and the post gain low so it does not squeal. I play my guitar hard (lots of powercords, heavy distortion) for just a little time (15 seconds) I can turn the post gain up to what ever volume I want and no squeal. Then I turned the pre gain down to play clean and there is still no squeal. After a little while (30 seconds) the signal starts to distort slightly then the squeal starts. I turn the post gain down a little and the squeal goes away and I can play for 15 seconds before the squeal starts again. I can keep turning it down a little to stop the squeal again and again until the post gain is down to where it originally would not squeal. I then can turn up the pre gain and play hard/distorted and then turn the post gain up again with no squeal.

I tried it again but when I turned it up with no squeal, I did not play anymore to see what would happen. After about 30 seconds I heard some noise getting louder. It sounded like noise (buzz) amps pick up from those halogen lamps with the dimmer knob/switch. (I have one of those lamps on in the other room so it probably is that) It was not real loud but you could hear it raise up out of the noise the amp normally makes. (since I put the pre-amp tubes in, there is a 60Hz hum that I was going to take care of after I work out the bugs) Then a hiss was next to rise up and join the buzz like someone was turning up a volume control. Then the squeal started.

When it is not squealing, the amp sound beautiful. I love it.

Update 2. I did some more searching and found some more things to try. I added a decoupling stage to the plates of the tubes. (TR11, TR12, TC6, and TC7) This helped. The intensity of the squeal is a lot less when you turn the volume up high, but it is still there. Then I tried lowering the gain by replacing plate resistors TR4 and TR8 with 120k and the cathode resistors TR3 and TR7 with 1.3k. No noticeable change in the squeal. Then I tried unbypassing the cathode resistors. Still squeals.

Playing the guitar through the amp now some things have changed. I can turn the post gain up about quarter way before the squeal starts. I now can not turn the post gain up to half before the squeal starts after I have been playing heavy/distorted like before. But if I back off the post gain, the squeal still takes a little while to start and I can turn it down to stop it. 5 seconds pass the it starts again, turn it down to stop it and 5 seconds it starts again. The noise/buzz that was rising up before the squeal is not noticeable anymore. Also I have discovered turning the treble pot up will stop the squeal too. Unless I turn the post gain up to about 3/4 max, then it starts again.

Another thing. I was trying the different change in update 2 and I turn the amp on to test. Turning up the post gain the squeal started but was a lot quieter, but no guitar signal was coming through. Then I noticed that I forgot to plug the speaker in. The squeal was coming from the ECC83 through the air!

I have tried changing tubes, but it stays the same. I have tried 2 JJ ECC83 and 2 Mullard old stock ECC83 and all squeal the same.